The present invention is concerned with prospecting and relates more particularly to a method of making for an area a map or a representation of local variations in the position of the geoid which have an amplitude less than about 1 m and are caused by density variations in the underlying sea floor, said map or representation being intended primarily for use in the determination of part areas of the sea floor with increased probability of deposits of natural resources of minerals and/or oil and gas bearing sediments. The density of which part areas distinguishes from that of the surroundings, downward bends in the geoid towards the sea floor indicating part areas having a density lower than that of the surroundings, and upward bends in the geoid from the sea floor indicating part areas having a density higher than that of the surroundings.
Hydrocarbon deposits sufficiently rich to justify exploitation occur in rock traps on land and below the sea floor. Sedimentary rocks containing these traps have a slightly lower density than the surrounding basement, which, apart from metamorphic rocks, also can consist of highly consolidated sedimentary rocks. This is one of the facts utilized in hydrocarbon prospecting operations.
For present day offshore hydrocarbon prospecting, use is made of a number of different techniques of which the following are the most important ones:
(a) Seismic surveying which involves the generating of acoustic impulses by means of an air or water gun and measuring of the acoustic signals reflected from certain geological markers below the sea floor by means of various types of sensors. The signals received fluctuate in time in response to the density variations in the different layers and formations below the sea floor and the depth to these layers and formations. The signals thus provide a geological picture indicating structures and main faults which could contain oil/gas accumulations.
(b) Magnetometric surveying which involves measuring the intensity and the direction of the terrestrial magnetic field.
(c) Gravimetric surveying which involves measuring minor variations in the vertical component of the gravitational field at the sea surface by means of an instrument which, in principle, comprises a spring-suspended sinker. The deflection of the instrument reflects the total force of gravity from all mass lying vertically between the location where the survey is carried out and the center of the earth. In other words, large mass densifications can contribute considerably to the deflection of the instrument even if they are at very large depths.
(d) Geological surveying and bottom samples by which it is intended, inter alia, to judge whether the conditions within the prospecting area during earlier geological times have been favourable to the formation of hydrocarbon-bearing areas, and it is investigated whether the rock is of the type in which oil/gas usually is to be found.
(e) Electrical surveying which involves investigating the character of the sea floor by resistivity measurements.
(f) Geochemical surveying. Hydrocarbon accumulations generally leak a certain amount of oil or gas to the overlying sediments and subsequently to the sea floor, and in some instances it is possible to trace and analyse these leaks by means of bottom samples.
All of these prior art techniques suffer from the disadvantage that they have to be performed physically on the area of investigation and thus that they become rather expensive in nature. Furthermore, the above techniques are not normally applied over large areas due to their costly nature and thus they do not possess any synoptic viewing. Since hydrocarbon prospecting by the above-mentioned techniques and, especially subsequent exploratory drillings are extremely expensive, large sums of money could be saved if the forecasting accuracy could be increased, in a way that areas of high probability for finds could be restricted prior to exploratory drillings.
In hydrocarbon and mineral prospecting on land, data are utilised which are collected by means of satellites for mapping areas with likely deposits. The techniques used for this purpose are colour television technique, picture analysis/processing and multispectral recording.
For off-shore applications such techniques are rendered useless, but, it is known that certain satellites are able to provide altimeter data, i.e. information about the distance between the satellite and the sea surface. A satellite revolves around the earth in a great circle plane. Since the earth rotates on its north-south axis, the satellite will gradually move back and forth within an area between specific north and south latitudes, the width of said area being determined by the orbital angle of the satellite. This area will eventually be scanned by the satellite which then will pass over the area along north-western and south-western tracks, crossing each other and forming a deformed grid pattern.
Altimeter data are measured at regular intervals along these tracks. At the crossing points of the tracks, the altimeter data will be recorded at different times for one and the same point. Because the tidal lift is different at different times, because the wind force is different, because the satellite is at different altitudes depending upon whether it has travelled over land or sea before it reaches the measuring point, etc., the altimeter data measured at different times will differ considerably at the crossing points. The differences may amount to .+-.5 meters.
By means of these altimeter data and information about the orbit of the satellite, it is then possible to calculate height values indicating the position of the sea surface in relation to a reference ellipsoid which is an imaginary ellipsoid representing the shape of the earth as accurately as possible based on the assumption that the earth is totally homogenous. These height values are used to study the undulation of the sea surface which is not entirely globular in shape, but slightly undulatory. In places where the force of gravity is stronger, the sea bulges slightly outwardly (water masses are attracted to these places), and in place where the force of gravity is somewhat less, the sea bulges slightly inwardly. The sea surface has, in other words, adapted itself to the difference in gravity pull along the surface of the earth. Further, if the sea surface is at complete rest it attains a surface with constant gravitational potential, a so-called geoid.
Up to now, the altimeter data have been utilised mainly for determining the position of the geoid, expressed in absolute numbers as height values above the reference ellipsoid. However, because of the difficulties involved in accurately determining the orbit of the satellite and in finding exact corrections for tides, waves, currents etc., only the variations in the geoid position which have a large spread and high amplitude could be determined. Scientific literature in this field gives examples of observations of variations in the geoid position of the order -40 m to +60 m. Variations of this order are entirely without interest for prospecting purposes and, besides, have not been utilised therefor. These variations essentially reflect the topography of the sea floor, the seamounts attracting water from the surroundings and raising the sea level, while the trenches lower the sea level, and have thus been utilised for mapping the sea floor topography.
Furthermore, it is generally accepted that there is isostatic equilibrium within large regions on the earth. This means that the mass of a column extending down to a fixed depth, below a certain area of the earth is the same as the mass of a column extending down to the same depth, below another, equally large area of the earth. If this were not the case, the column with the higher mass would sink relative to the column with the lower mass.
The Pratt-Hayford isostatic model suggests that the column have different densities for achieving the equilibrium.
The Airy-Heiskanen isostatic model proposes that all the columns have the same density, but have varying extensions into the denser mantle for achieving the equilibrium.
Two papers by Richard H. Rapp, "The Determination of Geoid Undulations and Gravity Anomalies from Seasat Altimeter Data", Journal of Geophysical Research, Vol. 88, No. C3, pp. 1552-1562, Feb. 28, 1983, and "Gravity Anomalies and Sea Surface Heights Derived from a Combined GEOS 3 Seasat Altimeter Data Set", Journal of Geophysical Research, Vol. 91, No. B5, pp. 4867-4876, Apr. 10, 1986, disclose a method of globally mapping the sea surface height expressed in absolute numbers above a reference ellipsoid. The method comprises obtaining height values which indicate the sea surface height in relation to a reference level and which have been calculated by means of altimeter data measured from a satellite and by means of information about the orbits of the satellite during measurement of the altimeter data; sorting out incorrect and improbable values; and adapting the height values corresponding to different orbits of the satellite to one another, such that maximum agreement of height values is obtained in the crossing points of the orbits. The values established for the variations in the sea surface height are, in addition, converted into variations in the gravity acceleration expressed in mGal.
It is the object of the present invention to determine local variations, i.e. variations less than 200 km in lateral extension, in the geoid position, which have an amplitude less than about 1 m and are caused by density variations in the underlying sea floor, and to make a map or representation of these variations, which can be used in prospecting for natural resources in the sea floor.